There are many types of electrical circuits that employ modulators. Communications circuits for transmitting and receiving communication signals, for example, typically include receiver or transceiver circuits having modulators. Such communication circuits may be used in a wide variety of devices and applications, including telephones, televisions, wireless computing devices, audio devices, personal data assistants (PDAs), and any other suitable systems that communicate signals. Of course, in addition to communications circuits, many other types of electrical circuits may include modulators.
Sigma delta modulators operating as continuous-time modulators, so-called continuous-time sigma-delta modulators, have the advantage of lower power consumption than sigma-delta modulators operating as discrete-time modulators. Alternately, a higher signal bandwidth may be achieved for the signal processing with the same power consumption, therefore making them attractive for all x-DSL Analog-Front-End transceivers. Continuous-time sigma-delta modulators are particularly suitable for wireless signal transmission devices over their discreet-time (DT) equivalents.
A block diagram of a continuous-time sigma-delta (CTSD) modulator 50 is shown in FIG. 1. The CTSD modulator 50 includes a continuous-time loop-filter 52 which represents the core of the modulator 50. An analog input 60 is applied to the continuous-time loop-filter 52, and a filtered analog output 62 is passed from the continuous-time loop-filter 52 to an analog-to-digital converter (ADC) 64. A filtered digital output 66 is transmitted from the ADC 64 of the CTSD modulator 50 to other components of the circuit or device. The filtered digital output 66 is also fed back to a digital-to-analog converter (DAC) 68, and is returned to the continuous-time loop-filter 52. In brief, the CTSD modulator 50 receives the analog input 60 and, using the continuous-time loop-filter 52, encodes the analog input 60 at clock intervals to provide the filtered digital output 66. The feedback of the filtered digital output 66 (provided to the DAC 68) is received as an analog signal back into the continuous-time loop-filter 52 where it may be used during the encoding. The structural and operational aspects of continuous-time loop-filters are described more fully, for example, in U.S. Pat. Nos. 7,151,474 issued to Ortmanns et al., 7,042,377 issued to Oliaei, 6,989,772 issued to Clara et al., and 6,396,428 issued to Cheng.
As depicted in FIG. 1, the continuous-time loop filter 52 can be implemented in several ways, such as by using resistor-capacitors 54, transconductor-capacitors 56, MOSFET-capacitors 58, or any other suitable components. In each of these different kinds of implementations, however, certain time constraints of the continuous-time loop filter 52 are typically defined by uncorrelated components.
A general drawback with the CTSD modulator 50 is that filtering coefficients used within the continuous-time loop-filter 52 vary during performance of the CTSD modulator 50. In some cases, a wide range of process spread can cause the coefficients to vary by up to +/−40%. Such variations typically cause a performance loss by the CTSD modulator 50 and, when the filter design is very aggressive (i.e. in high order systems with low over-sampling ratio), may also cause unstable performance.
When the CTSD modulator 50 is embedded in a more complex system, such as a communication circuit of a communication device, it may be desirable to provide an automatic tuning technique that sets the coefficients to desired or optimal values either once at the beginning during the start-up phase, or adaptively in a continuous background process. Such tuning techniques have conventionally required considerable additional circuitry overhead, and may also require a replica of the overall complex system with which to conduct trial-and-error evaluations to empirically determine the desired coefficient values. Novel tuning techniques that mitigate these undesirable aspects of the prior art would have considerable utility.